1. Field of the Invention
The present invention relates to data communications. It is particularly suitable for power line communications (PLC) between locations having a common electrical distribution system.
2. Description of the Related Art
PLC, also known as Broadband Power Line (BPL), is a technology that encompasses transmission of data at high frequencies through existing electric power lines, i.e., conductors used for carrying a power current. Power current is typically transmitted through power lines at a frequency in the range of 50–60 hertz (Hz). In low voltage lines, power current is transmitted with a voltage between about 90 to 600 volts, and in medium voltage lines, power current is transmitted with a voltage between about 2,400 volts to 35,000 volts. The frequency of the data signals is greater than or equal to about 1 Megahertz (MHz), and the voltage of the data signal ranges from a fraction of a volt to a few tens of volts. Data communication can employ various modulation schemes such as amplitude modulation, frequency modulation, pulse modulation or spread spectrum modulation.
A basic element of PLC technology is an inductive coupler for coupling PLC signals to and from a power line. Inductive coupling is most effective where the RF impedance of the power line is minimized.
Since current can only flow through a closed circuit, or loop, a signal current flowing from one point to another over a wire must have a “return path” to close the loop. When power line communication between two locations is desired using an inductive coupler at each location, a return path impedance at radio frequencies should be minimized. In a power line topology in which a single conductor, i.e., wire, is used, the return path impedance includes the impedance of the wire between two locations, plus the sum of all other impedances in the return path. The impedances in the return path, including the RF impedance of shunt devices helping to complete the return path, may be high, relative to the inherent impedance of the wires themselves. A high RF impedance reduces the magnitude of signal current induced by an inductive coupler, thus increasing the signal attenuation between the two locations.